The problem with non-member tourneys is that there are many people who sign up to the site, try it and the leave. Obviosly these people are deluded as no one sane would stop visiting here, but they do exist.
A tourney takes a long time to complete and the likely hood is that most matches would eb abandoned because or both players had left the site. I think th eidea is that someone who is serious enough to stick around for the 6 months+ a tourney takes is generally serious enough to subscibe.
Despite this I believe that there is a non-members tourney being trialed at the moment. Let's se how that one oges.